The present invention relates generally to the manufacture of electronic devices. More particularly, the present invention relates to the fabrication of T-gate structures used in the manufacture of electronic devices.
A T-gate is a gate conductor structure for a semiconductor device in which the top of the gate conductor structure is wider than the base of the gate conductor structure. Such T-gates include, without limitation, structures that are substantially T-shaped, mushroom-shaped, and Y-shaped.
In general, gate structures such as T-gates have been advantageously used in several technologies. For example, metal semiconductor field effect transistors (“MESFETs”), high electron mobility transistors (“HEMTs”) (variant of gallium arsenide field effect transistor technology) mainly used in satellite broadcasting receivers, high speed logic circuits and power modules have employed gate structures with bases smaller than the contact area. These types of gate structures are required in field effect transistors for operation in ultra-high frequency ranges. The narrow base of a T-gate structure provides a short channel length which results in increased speed and decreased power consumption. Parasitic resistances and capacitances that limit device speed are also reduced. The top portion of a T-gate is made wide so that the conductance of the T-gate remains high, for example, for high switching speeds.
Recent advances in CMOS transistor architecture make use of T-gate structures where the polysilicon gate electrode is narrowed in the gate regions and wider on top of the gate. This is due to the ever increasing demand for scaling down semiconductor devices and scaling down power consumption requirements.
Electron-beam (“e-beam”) is the most commonly used technique for T-gate fabrication. FIGS. 1A-1D illustrate a process for forming a T-gate using e-beam. Typically, substrate 1 is coated with a layer of first poly(methyl methacrylate)-based photoresist 2, a layer of second poly(methyl methacrylate)-based photoresist 3, and a layer of third poly(methyl methacrylate)-based photoresist 4. Photoresist layers 2 to 4 are then exposed to e-beam and developed to provide a patterned photoresist stack having generally T-shaped profile 5 as shown in FIG. 1B. A layer of a conductive material 6 is then deposited on the entire surface inclusive of the surface of substrate 1 exposed by the patterning of the photoresist layers, see FIG. 1C. Photoresist layers 2 to 4 are then removed, lifting-off the conductive material layer on the surface of photoresist layer 4 in the process, to provide T-gate structure 7 on substrate 1 as shown in FIG. 1D.
However, such e-beam techniques suffer from certain drawbacks. For example, e-beam lithography suffers from poor linewidth control in the multi-layered stacks used in typical T-gate processes because the exposing e-beam must pass through relatively thick resist films (e.g., about one micron). Further, e-beam exposure is a direct write process which is both slow and expensive.
Other methods of forming T-gates have been developed. Certain of these methods utilize a number of sacrificial inorganic layers which require various etching steps and harsher removal processes than photoresist-based processes. Other methods utilize multiple photoresist layers, however, these multiple photoresist layers are imaged at different wavelengths. For example, U.S. Pat. No. 6,387,783 (Furukawa et al.) disclose a process for forming T-gates using a hybrid first photoresist that is imaged using x-rays and a second photoresist that is imaged using I-line radiation. The use of such different wavelengths requires different exposure tools, which increase the costs and complexity of the process. Accordingly, a need exists for improved methods of forming T-gate structures.